The present invention generally relates to an integrated circuit device having a signal discrimination circuit, and more particularly to an improvement in testing the signal discrimination circuit by use of a very small voltage. Further, the present invention relates to a method of testing the integrated circuit device.
An integrated circuit device having a signal discrimination circuit is known. For example, a signal discrimination circuit is formed by an emitter coupled logic circuit which determines an output logic based on the potential relationship between a pair of input signals, namely a data signal and a reference signal. When testing such a signal discrimination circuit, it is required to supply the signal discrimination circuit with a pair of test signals having a very small potential difference.
When testing a signal discrimination circuit as described above in a step of production of an integrated circuit device having the signal discrimination circuit, an automatic IC tester having a power source device for use in test, such as a "4145B" tester marketed by YHP or a "T3340" tester marketed by Advantest typically is used. The automatic IC tester can generate a pair of test signals having a very small potential difference. The generated test signals are applied to input terminals of the signal discrimination circuit, and logic levels derived from the signal discrimination circuit are monitored. Then it is determined whether the signal discrimination circuit can operate correctly on the bases of the monitored logic levels.
The aforementioned IC tester "4145B" has the following specification:
(i voltage range: .+-.20 V PA1 ii) precision: .+-.(0.1% +10 mV +0.4.OMEGA..times.Io) where Io is an output current. The aforementioned IC tester "T3340" has the following specification:
______________________________________ i) voltage range: .+-.2 V, .+-.8 V ii) precision: for .+-.2 V, .+-. (0.2% + 2 mV + 0.6 mV/10 mA) for .+-.8 V .+-. (0.2% + 4 mV + 0.6 mV/10 mA) ______________________________________
However, the aforementioned method of testing by such testers has the following disadvantages. The very small test signals are directly supplied to the input terminals of the signal discrimination circuit to be tested. Thus, the test is influenced by any absolute error of the power source device built in the IC tester. Generally, the absolute error of the typical power source device is approximately equal to 2 mV. This value of the absolute error is substantially constant and independent of the magnitude of the output voltage derived from the power source device. When it is assumed that the output voltage derived from the power source device is equal to 10 mV, the ratio of the absolute error to the output voltage is 20% (=(2 mV/10 mV).times.100%). Thus, the testing precision is subject to a 20% error. It is noted that even a highly regulated power source device contains an absolute error in the output signal, without exception. For this reason, when such a power source device is used in a very narrow voltage range, the ratio of the absolute error to the output is large and thus the testing precision is poor.